Injection Moulding design - Drafts, Part Thickness & Tolerances

At Toolcraft we have longstanding experience of designing plastic parts for the injection moulding process. This short guide covers draft angles, thin & thick part design and tolerances. We also share a cost-saving design tip: designing by halves.

Draft angles

Finish

Min. angle

Polished finish

0.5°

Fine spark

3°

Medium spark

5°

Rough spark

10°

In injection moulding, you'll always need to keep the draft angle in mind, especially if you're designing a product with a textured surface. Basically, the rougher the surface, the larger your draft angle will have to be, otherwise the texture will prevent the part from coming out of the mould easily, or the part may be damaged. View the table to the right for an indication of the required draft angles for surface texture.

Thin features

If you need a very thin or delicate feature, it's probably best to have some models of your design made first. If the SLA/SLS model doesn't show up your feature adequately, it is unlikely that you'll get a better result with the plastic moulding process. Increase the size or thickness of the feature and have some more models done to ensure the feature reproduces correctly, before investing in an injection moulding tool.

Thick features

Here's a nice & simple rule: the 2/3 rule. If you need a boss or rib on an injection moulded product, the width of the boss shouldn't be more than 2/3 of the thickness of underlying wall. Otherwise, you'll get sinking of the plastic material underneath the feature. A way to get around this problem is to core out thick areas.

Tolerances

Don't specify too many areas that will need a tight tolerance. It's very difficult to achieve them consistently and unless you really need them, they'll prolong the design & modelling process, drive the price of your product up unneccesarily and perhaps even make it impossible to design.

It's also not practical to specify the same tolerance across all types of mouldings & materials, due to the vast range of materials available. As a guide, BS7010:1988 is a list of achievable tolerances for each material, available from the BSI website (link opens in new window). Toolcraft far exceed the BSI tolerances through correct tool design & optimisation !

Toolcraft Tip – Designing by Halves

Are you working on a project that would need tooling with withdrawing parts, which will make he mould tool quite expensive ? Consider designing it in 'two halves' and using ultrasonic welding to join the halves together afterwards. This can mean considerable savings on tooling costs. For an example of how Toolcraft applied this, have a look at our Golf-Tech editorial.

We have a lot more plastic moulding advice for you to explore, or if you're ready to benefit from our excellent design service, just